Beloitoceras sinuososeptatum ( Roemer, 1861 )

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 17-20

Beloitoceras sinuososeptatum ( Roemer, 1861)

Figs 9 A-B, 11

Orthoceras sinuoso-septatum Roemer, 1861 : pl. 6, fig. 3a-c.

Beloitoceras heterocurvatum Strand, 1934: 76 , pl. 10, figs 8-9.

Richardsonoceras sinuososeptatum – Dzik 1984: 188, text-fig. 16a-b, pl. 8, fig. 1a-d.

Beloitoceras heterocurvatum – Frye 1987: 84, figs 3a-d. ― Kröger et al. 2011: 38, fig. 5a-b.

Emended diagnosis

Beloitoceras with compressed cross section with keeled conch margins at concave and convex side of conch curvature throughout entire length; in longitudinal section antisiphuncular conch margin concave; angle of expansion of phragmocone ca. 16–17°; depth of body chamber somewhat less than maximum conch height; conch surface ornamented with irregularly spaced rounded striae, which form a shallow sinus on prosiphuncular side and deep v-shaped sinus on antisiphuncular side; about six chambers

similar to the height of the conch cross section; sutures with deep lateral lobes; siphuncle situated near ventral margin, segments ca. twice as high as wide, with slightly convex vertical outline.

Type locality and horizon

Zawidowice near Oleśnica, Poland, erratic boulder of probably latest Katian age, Ordovician.

Material examined

Thirteen specimens in total: PMU 26633–26642, 24769, 24772, Kallholn, Dalarna; one specimen ( PMU 24770), Skålberget, Dalarna, Boda Limestone, late Katian.

Description

The general features of the conch, the ornamentation and the details of the siphuncle were described by Strand (1934), Dzik (1984) and Frye (1987). These descriptions were based on relatively few specimens. Here, 15 additional specimens are available for measurements and direct comparison. These specimens reveal a high variability in conch curvature and angle of expansion. The conch curvature varies from strongly curved to relatively widely curved ( Fig. 9 A-B). The angle of expansion varies from 15° (1 st quantile) to, 18° (3 rd quantile) with a mean of 17° (15 measurements) in the specimens from the Boda Limestone. The conch cross section is always compressed with a typical keel at the anti-, and prosiphuncular side, but the rate of compression varies among the Boda Limestone specimens from 0.85 (1 st quantile) to 0.92 (3 rd quantile) with a mean of 0.87 (10 measurements). Within this range of variance no distinct groups of specimens can be distinguished ( Fig. 11 ).

Remarks

In earlier descriptions the variability of the conch shape of this species was not recognized. Additionally, the adult size of different specimen assigned to this species varies from 28 to 38 mm maximum body chamber height. Because only three specimens from the Boda Limestone possess an adult body chamber (specimen figured in Frye 1987: fig. 3a, b; PMU 24769, 26634), it is impossible to evaluate whether this difference reflects interspecific variability, sexual dimorphism or the presence of more than one species.

The type specimen from Vormsi/Pirgu Regional Stage limestones of the erratic boulders of northern Poland, figured by Dzik (1984, pl. 8, fig. 1a-d), differs from the Boda Limestone specimens in its exceptionally low angle of expansion of only 10°. Moreover, the paratype figured by Dzik (1984), with a width/height ratio of 0.72, and the specimen from the Oslo area figured by Strand (1934: pl. 10, fig. 8) are considerably more compressed than the specimens from the Boda Limestone. Also, these two specimens represent the lower range of the angle of expansion for all the specimens measured ( Fig. 11 ). However, based on the available information it is impossible to distinguish different species of Beloitoceras among the specimens figured and described by Dzik (1984: pl. 8, fig. 1a-d), Strand (1934), Frye (1987: fig. 3a, b) and the material described herein. Based on the available material, B. sinuososeptatum is considered a species of Beloitoceras with a highly variable conch curvature, high variance in the angle of expansion ( Fig. 11 ), and potentially encompasses macro-, and microconch varieties.

An apical fragment, PMU 26646, is interpreted as Beloitoceras sp. ( Fig. 9 E-F), because of its similarity in general conch shape and ornamentation with the larger fragments. The apex is cup-shaped and reaches a conch height of 4.3 mm and a width of 4 mm at a distance of 2 mm from the tip. The shell of this specimen is ornamented with fine rounded transverse annuli (ca. 8–10 per millimetre), which form a shallow hyponomic sinus on the concave side of the curved conch.

Comparison

Two groups of Beloitoceras are distinguishable within the Boda Limestone. According to Frye (1987), B. siljanense Frye, 1987 can be distinguished from B. sinuososeptatum in having an antisiphuncular conch margin that straightens relatively early during growth and remains straight in the mature body chamber. By contrast, in B. sinuososeptatum the antisiphuncular conch margin is entirely curved along the entire length of the concave side of the conch. It was emphasized above that the conch curvature and angle of expansion vary strongly between specimens of Beloitoceras from the Boda Limestone. Within this continuous spectrum of variation, the types of both species represent two extremes with a maximum conch expansion in B. siljanense Frye, 1987 (23°) and a minimum in B. sinuososeptatum (10°). Based on the angle of expansion alone, it is not possible to distinguish between the two species ( Fig. 11 ). B. siljanense may be distinguished from B. sinuososeptatum in having a finer transverse ornamentation and a less curved antisiphuncular conch margin (compare Fig. 9 A-B, C-D).

Beloitoceras landerense Foerste, 1935 from the mid Katian Lander Sandstone of Wyoming, USA is very similar to B. sinuososeptatum in general conch shape and dimensions ( Fig. 11 ). As the details of the ornamentation of this species are not known, direct comparison and synonymisation with B. sinuososeptatum is not possible at present.

Beloitoceras whitneyi (Hall, 1861) from the Elgin Formation, late Katian, Iowa, USA is similar to B. sinuososeptatum in general conch shape, its transversely ornamented shell, and in possessing a relatively large adult conch height of ca. 35 mm. It may be conspecific with the Boda specimens, but with ca. 9–10 chambers in a length similar to the corresponding conch height, the chamber distance (cameral depth) is significantly lower in Beloitoceras whitneyi than that of B. sinuososeptatum (six chambers per length similar to corresponding conch height) and nothing is known about the variability of this character in either species. A synonymisation of this species is very likely, but may only be possible to demonstrate after comparison of the variability of the chamber height in both assemblages.

Stratigraphic and geographic range

Bønsnes Formation, Ringerike district, Skogerholm Formation, Oslo-Asker district, Norway ( Strand 1934); erratic limestone boulders from Vormsi/Pirgu Regional Stage (“Lyckholm stage”), Zawidowice, Poland ( Dzik 1984); Vormsi/Pirgu Regional Stage (“Lyckholm stage”), Estonia ( Teichert 1930); Boda Limestone, Dalarna, Sweden; all late Katian.

Figures

Fig. 9. Oncerida of the Boda Limestone. Note the high variability in conch curvature and angle of expansion in Beloitoceras. A-B. Beloitoceras sinuososeptatum (Roemer, 1861). A. PMU 24769. B. PMU 26638, Kallholn. C-D. Beloitoceras siljanense Frye, 1987. C. PMU 26644. D. PMU 26645, Kallholn. E-F. Apex of Beloitoceras sp., PMU 26646, Kallholn. E. Adapical view. F. Lateral view. G-H. Cyrtorizoceras thorslundi sp. nov., PMU 26657, Osmundsberget, mature body chamber. G. Lateral view. H. Adapical view. I. Beloitoceras siljanense Frye, 1987, PMU 26643, Kallholn, with major conch repair at adoral part. J. Cyrtorizoceras thorslundi sp. nov., PMU 26647, holotype, Kallholn, complete juvenile specimen. Scale bars: A-D, G-J = 10 mm; E-F = 2 mm.

Fig. 11. Morphological variation of Beloitoceras sinuososeptatum (Roemer, 1861). Circles: specimens of the Boda Limestone. Black dot: type specimen of Beloitoceras siljanense Frye, 1987. Black diamonds: types of Beloitoceras heterocurvatum Strand, 1934. Grey dot: type specimen of Beloitoceras landerense Foerste, 1935. A distinction between B. siljanense and B. sinuososeptatum is impossible based on the available material.